Shifts of the 1A−g→1B+uelectronic absorption of carotenoids in nonpolar and polar solvents

The 1B+u energy of carotenoids (spheroidene, neurosporene, and β carotene) exhibits a linear dependence on R(n)=(n2−1)/(n2+2) in both nonpolar and polar solvents: The 1B+u energy is more stabilized in polar solvents in the limit of R(n)→0, and the line for polar solvents has a gentler slope. [As a result, the line for polar solvents crosses the line for nonpolar solvents at R(n)≂0.3.] A theory has been developed, which explains the aforementioned observations as follows. In polar solvents, an electric field is generated by fluctuation of the solvent permanent dipoles, and it affects the rodlike, conjugated chain of the carotenoid in a long spheroidal cavity. The electric field stabilizes the 1B+u energy through the polarization effect and it substantially reduces the dispersive interaction. The higher transition multipoles, rather than the transition dipole of carotenoid, play major roles in the aforementioned interactions between the carotenoid and the solvent molecules.